This invention relates to a method of manufacturing a liquid crystal display device.
A liquid crystal display device is generally manufactured by disposing a pair of insulating substrates in parallel relationship at a definite spacing and then sealing the periphery of the substrates with a peripheral sealing member such as frit glass or solder glass except a small inlet opening for filling with a liquid crystal, thus forming a liquid crystal cell. At least one of the substrates is transparent for displaying patterns and, a flat transparent electrode is formed on the inner surface of the transparent insulating substrate. After filling the liquid crystal into the cell through the inlet in a well-known manner, the inlet opening is sealed by a liquid crystal inlet sealing agent to complete a liquid crystal display device.
Since a liquid crystal is liable to be affected by moisture, oxygen in the air, and heat, and is chemically active, the peripheral sealing member is required to be impervious to water. Moreover, as organic substances can not satisfy these requirements, in inorganic substances such as frit glass is used. However, after the liquid crystal has been filled in the cell, it is impossible to use frit glass to seal the liquid crystal inlet opening. Because it is necessary to use an elevated temperature of order of 400.degree. C to 500.degree. C for the treatment of the frit glass, the liquid crystal decomposes at a temperature within the above range. For this reason, it is necessary to use an inorganic sealing agent other than frit glass for sealing the liquid crystal inlet opening. A method of sealing has been proposed wherein a solder containing a small amount of transition elements and sold under a trade name of Cerasolzer is directly applied to the glass by ultrasonic soldering technique. However, this method is defective in that fine particles of the solder enter into the liquid crystal thus forming a black sludge or causing short circuit of the electrodes. This, of course, impairs patterns to be displayed.
According to another method, a paste containing Ag-Pd, Au-Pd or the like is backed on the area surrounding the inlet opening and then soldering is effected. However, this paste requires a baking temperature of 500.degree. C to 650.degree. C thus degrading the liquid crystal. If baked at a lower temperature not to soften the peripheral sealing member comprising frit glass, bonding strength would be deficient.
To solve these problems a method has been proposed wherein a metal film is vapour deposited onto the area surrounding the liquid crystal inlet opening and then the opening is sealed with an eutectic solder consisting of 63% of tin and 37% of lead which solidifies, upon cooling, in an eutectic state. When a mixture (or alloy) of nickel and chromium is vapour deposited about the liquid crystal inlet opening, a metal film is formed containing chromium having lower evaporation temperature at the inner portion and nickel having higher evaporation temperature at the outer portion. Chromium has a strong bonding strength to glass. The bonding strength of nickel to glass is not so high but it can be soldered satisfactorily thereby enabling the application of the eutectic solder on the vapour deposited metal film. When a film of gold is formed on the metal film, more satisfactory soldering can be made and oxidation of the metal film can be prevented.
However, when the soldering operation is performed in the air, the surface of the resulting solder film becomes irregular containing many projections and depressions, the largest projection being formed at a point where a soldering iron is removed from the soldered film. This is caused by the fact that the surface of the molten solder is oxidized and increases its viscosity more than in the inner side. The molten solder having an increased viscosity tends to follow the escaping soldering iron. As will be described later with reference to the accompanying drawing, such large projection will rupture a part of the insulating substrates close to the soldered portion sealing the liquid crystal inlet, on account of difference in thermal expansion coefficient between the insulating substrate and the solder. Moreover such irregular surface decreases the dimensional accuracy of the liquid crystal device.